Last week, in a civil service, the writer Jan Morris remarried the wife she first married as a man. In 1949, James Morris, a young journalist, married Elizabeth Tuckniss, daughter of a tea planter. "I have lived with the same person for 58 years," Morris told reporters. "We were married when I was young. . . and then this sex-change, so-called, happened, so we naturally had to divorce ... but we always lived together, anyway. So, I wanted to round this thing off nicely. So last week, as a matter of fact, Elizabeth and I went and had a civil union."

Elizabeth Morris, who has stood by her man with a commitment not envisaged in Tammy Wynette's philosophy, said: "After Jan had a sex change we had to divorce. So there we were. It did not make any difference to me. We still had our family. We just carried on."

At a time when many relationships are so fluid they wouldn't survive a change of wind direction, it's lovely to read that some relationships endure - against all the odds, against whatever anybody thinks. "To catch a husband is an art; to hold him is a job," wrote Simone de Beauvoir.

Especially if he turns into a wife, she might have added.

But de Beauvoir is wrong: it's not a job to hold on to a spouse, more of a feat, often incomprehensible to outsiders. Hugh Hefner and Kimberley Conrad, Jean-Paul Sartre and de Beauvoir, Joseph and Magda Goebbels, Frida Kahlo and Diego Rivera; the list of relationships that have endured unlikely circumstances is longer than you might think.

Take Harold Nicolson and Vita Sackville-West, who remained devoted to each other even though both had same-sex affairs during their long marriage. Their marriage survived Sackville-West's elopement to France with Violet Trefusis and Virginia Woolf presenting her with the novel Orlando, which Sackville-West's son Nigel Nicolson described as "the longest and most charming love letter in literature". Still, they wrote to each other every day when they were apart and together created the gardens at Sissinghurst Castle.

Think, too, of Tom Jones and Linda Trenchard. Linda who, you might ask? She's the woman whose name often appears after the adjective "long-suffering", since the venerable Welsh sex bomb has had many affairs during their 51-year marriage. What's their secret? What's the secret for Morris and Tuckniss? You know what - it's none of our business. Enough, surely, that their marriage has endured. Enough that some mysteries remain just that.